OBJECTIVES: 1. Further studies on synapse formation between rat primary muscles and spinal cord explants. Frequency of miniature endplate potentials (MEPPs) was surprisingly high (up to a few/s) in newly formed synapses when the small synaptic area was taken into consideration. MEPP amplitudes did not increase during culture period up to 2 weeks. Correlations between developmental changes in ultrastructures and electrophysiological properties in the synapses was studied. Accumulation of synaptic vesicles to an electron dense spot in the nerve terminal confronting the myotube membrane was found in the neuromuscular junction formed 2 days after spinal cord explants were added to the myotubes. Number of vesicles increased during culture period. The postsynaptic membrane thickening was ocasionally found. Subsynaptic foldings were developed in old cultures (10 to 14 days of co-culture). 2. Correlation between MEPP amplitudes and acetylcholine (ACh) receptor densities was studied in rat primary myotubes and myotubes from a rat muscle cell line (H9). Mean MEPP amplitudes in these myotubes were proportionally related to the receptor densities before innervation. 3. MEPP amplitude distribution and ACh receptor distribution were correlated in Xenopus neuromuscular junctions formed in vitro. 4. Functional cholinergic synapses were found between a rat pheochromocytoma cell line and a rat skeletal muscle cell line. BIBLIOGRAPHIC REFERENCES: Brandt, B.L., Hagiwara, S., Kidokoro, Y., and Miyazaki, S. (1976). Acton potentials in the rat chromaffin cell and effects of acetylcholine. J. Physiol., 263, 417-439. Schubert, D., Heinemann, S., and Kidokoro, Y. (1977). Cholinergic metabolism and synapse formation by a rat nerve cell line. Proc. Natl. Acad. Sci. USA, in press.